Putting Slime Pits, Cursed Oasis, Ick Swamp, Labyrinth, and Shifting Passages into their own "very hard" category is sensible. Not sure if I'd put HHoS there; it's no more cramped than Den of Danger, its bosses less threatening than Creeplight Ruins, and its magic-heavy monster roster trivialized by the Orb of Lusory.

Part of the issue is that the difficulty labels have specific technical meaning aside from the subjective impression of how hard they are. EASY, NORMAL, HARD, and VICIOUS all refer to the stat array that the monsters use. The only exception that I know of is Grim's Grotto, which is labeled as HARD but has a unique array that is slightly above "normal" HARD.

What I was actually interested in was an easy way to reffer to them, as in, a standard term. Writing "penultimates" every time or 2nd HARD, or "the ones before VICOUS" or pre-vicious - what'd be the best and easiest?

Not to derail this, but the 8 lvl1 mob subdungeon in CO is so cruel - "look at all these mobs you can't remove your curse on!" I agree CO looks tamer once you beat it a few times. Now Slime Pits, ugh...